1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an object image.
2. Related Background Art
Conventional solid-state image pickup elements are mainly classified into CCD-type elements and CMOS-type elements. Examples of CCD-type elements are an interline CCD (IL-CCD) and frame transfer CCD (FT-CCD).
An interline CCD comprises a plurality of photoelectric conversion units, vertical CCDs, transfer gates for transferring signal charges in the photoelectric conversion units to the vertical CCDs, a horizontal CCD for horizontally transferring the charges from the vertical CCDs, and a floating diffusion amplifier (FD-Amp). A frame transfer CCD comprises photoelectric conversion units, a storage unit to which charges in the photoelectric conversion units are transferred (frame-shifted), a horizontal transfer unit for horizontally transferring the charges from the storage unit, and a floating diffusion amplifier (FD-Amp). FIG. 11 shows an IL-CCD as a typical example of a CCD-type solid-state image pickup element.
As shown in FIG. 11, an IL-CCD comprises photoelectric conversion units (PDs) 70, vertical CCDs (V-CCDs) 71 for temporarily storing charges from the photoelectric conversion units 70, a horizontal CCD (H-CCD) 72 for receiving the charges from the vertical CCDs 71 and transferring the charges in the horizontal direction, and a floating diffusion amplifier (FD-Amp) 73 serving as an output amplifier for converting the charges from the horizontal CCD 72 into a voltage.
The CCD has low noise level because signal charges are completely transferred to the FD-Amp on the output side. In addition, the pixel size can be made small because of the simple pixel structure. Today, however, the number of pixels in a sensor for a digital camera reaches several millions for higher quality, and high-speed drive is indispensable in an HD camera. For this reason, the power consumption of a horizontal CCD drive circuit is high (up to CfV2). Although a CDS circuit for reducing reset noise in an FD-Amp requires a sampling accuracy on the order of nsec, that sampling accuracy cannot be ensured because of the mass production and the wide range of use conditions (temperature and a variation in voltage). Hence, a low-noise CCD can hardly be implemented.
In a CMOS sensor, as shown in FIG. 12, each pixel unit 80 comprises a photoelectric conversion unit PD and a MOS transistor serving as a pixel amplifier. The pixels are selected and driven by a random-accessible vertical scanning circuit 81 and horizontal scanning circuit 85, resulting in functional advantage. Charges in each pixel unit 80 are converted into a voltage by the pixel amplifier, output to a clamp circuit 83 through a vertical signal line 82, and after noise removal from the pixel unit amplifier by the clamp circuit 83, output to a horizontal signal line 84. The signals are sequentially output by the horizontal scanning circuit 85 from the horizontal signal line 84 through an amplifier (Amp) 86. Since a CMOS sensor sequentially selects and outputs each pixel signal by the horizontal scanning circuit 85, unlike charge transfer as in a CCD, the power consumption is low. However, since the number of elements of each pixel unit is large, it is difficult to reduce the pixel size.
As described above, a CCD is advantageous in increasing the number of pixels but disadvantageous in high-speed drive and power consumption. Conversely, a CMOS sensor is disadvantageous in increasing the number of pixels but advantageous in high-speed drive and power consumption.